Ophthalmic lenses used in eyeglasses can be designed to correct for any of a number of vision defects, including nearsightedness (myopia), farsightedness (hyperopia), astigmatism, and reduced ability of the eye's crystalline lens to accommodate, i.e., to focus on nearby objects (presbyopia). People with a combination of contrary vision impairments, e.g., people suffering from both presbyopia and either myopia or hyperopia, are often prescribed lenses with two or more viewing zones (i.e., lens portions designed to provide certain visual aids to the wearer) of different refractive powers (also referred to as “dioptric” powers) that compensate for these different defects. In conventional multi-focal lenses, the transitions between the various viewing zones are abrupt, whereas in progressive lenses, they are smooth.
Examples of multi-focal lenses are conventional bifocal lenses, which often have a lower refractive power (caused, e.g., by a less convex front surface) in the upper half to facilitate distance vision, and a higher refractive power (caused, e.g., by a more convex front surface) in the lower half, or a portion thereof, to aid in close viewing. Modern bifocal lenses typically reserve the main portion of the lens for distance vision, and a smaller portion located in the lower half for near vision. The smaller portion for near vision is often referred to as the “reading segment,” “add segment,” or simply “segment.” FIG. 1A illustrates schematically a lens 100 with a round reading segment 102. Trifocal lenses include three zones for distance, intermediate (i.e., about arms-length), and near vision. In general, multi-focal lenses may have any number of corrective zones with abrupt transitions between the zones.
An alternative to multi-focal lenses (in particular, lenses containing multiple zones with different constant dioptric powers) are lenses whose dioptric power varies continuously within at least a portion of the lens and which, consequently, feature smooth transitions between the different viewing zones. Such “progressive” lenses may feature a first surface (e.g., the back surface) having a constant radius of curvature and a second surface (e.g., the front surface) having a variable radius of curvature, or vice versa. For example, the front surface may have a constant large radius of curvature in an upper zone, a constant small radius of curvature in a lower zone, and a radius of curvature that varies gradually between the large and the small radius in a middle zone. This middle zone may enable intermediate-distance viewing. In general, progressive lenses need not contain spherical portions having a single radius of curvature, but may, for example, have portions with two different radii of curvature. Examples include lenses with toric or cylindrical power distributions (e.g., lenses having different horizontal and vertical radii of curvature). Progressive lenses have aberrations (e.g., an undesired astigmatism in the periphery of the intermediate progressive zone) associated with them, which may significantly reduce the field of correct vision. In general, the distortions are greater (and more annoying to the wearer), as the range of dioptric powers increases in the region spanned by the progressive zone. FIG. 1B illustrates schematically a progressive lens 120 having dioptric power that varies continually along a central meridian in a middle zone. The lens provides only a narrow region 122 of correct intermediate vision, which is flanked by regions 124 of high distortion.
Ophthalmic lenses may combine segmentation into multiple zones with progressive zones to improve the size and/or quality of the various viewing zones. For example, an ophthalmic lens may include a main portion with constant dioptric power for distance viewing, and an add segment with a progressive zone for intermediate-distance viewing that merges into a constant zone for near viewing. The segmented lenses generally have surface slope discontinuities across the boundaries between the zones, rendering the boundaries clearly visible. This aesthetically undesirable effect may be mitigated by “blending” the zones around their boundaries, i.e., smoothening the transitions between the zones. Such smoothening results in an interjacent zone along the boundary, in which the surface gradient varies gradually between the surface gradients on either side of the boundary. FIG. 1C schematically depicts a lens 140 featuring an add segment 142 with a blended boundary 144. Across the blended boundary, the dioptric power varies significantly, and generally both discontinuously and non-monotonously. Thus, while blending may render the boundary nearly invisible, it introduces vision distortions within the blended region.